Visualization of a water-selective pore by electron crystallography in vitreous ice

The water-selective pathway through the aquaporin-1 membrane channel has be en visualized by fitting an atomic model to a 3.7-Angstrom resolution three -dimensional density map. This map was determined by analyzing images and e lectron diffraction patterns of lipid-reconstituted two-dimensional crystal s of aquaporin-1 preserved in vitrified buffer in the absence of any additi ve. The aqueous pathway is characterized by a size-selective pore that is a pproximate to4.0 +/- 0.5 Angstrom in diameter, spans a length of approximat e to 18 Angstrom, and bends by approximate to 25 degrees as it traverses th e bilayer. This narrow pore is connected by wide, funnel-shaped openings at the extracellular and cytoplasmic faces, The size-selective pore is outlin ed mostly by hydrophobic residues, resulting in a relatively inert pathway conducive to diffusion-limited water flow. The apex of the curved pore is c lose to the locations of the in-plane pseudo-2-fold symmetry axis that rela tes the N- and C-terminal halves and the conserved, functionally important N76 and N192 residues.